Add the project skeleton and the approved design

Cargo.toml with the release profile and lints, the toolchain pinned to
Rust 1.90.0, a build script that records the git commit for
`cumulus --version`, and rustfmt settings and cargo aliases (`cargo lint`,
`cargo linux`) as in skyshift. DESIGN.md holds the design approved after
the question rounds; README.md has build instructions and the planned keys.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

rust-toolchain.toml

@@ -0,0 +1,8 @@
+# The Rust version cumulus is built with. rustup reads this file and installs
+# this exact version, with these extras, the first time it's needed.
+[toolchain]
+channel = "1.90.0"
+profile = "minimal"
+components = ["clippy", "rustfmt"]
+# For `cargo linux` (see .cargo/config.toml).
+targets = ["x86_64-unknown-linux-musl"]